1. Field of the Invention
The present invention relates to self-cleaning surfaces, and to processes for their production.
2. Discussion of the Background
Objects with surfaces which are extremely difficult to wet have a number of commercially significant features. The feature of most commercial significance here is the self-cleaning action of low-wettability surfaces, since the cleaning of surfaces is time-consuming and expensive. Self-cleaning surfaces are therefore of very great commercial interest. The mechanisms of adhesion are generally the result of surface-energy-related parameters relating to the two surfaces which are in contact. These systems generally attempt to reduce their free surface energy. If the free surface energies between two components are intrinsically very low, it can generally be assumed that there will be weak adhesion between these two components. The important factor here is the relative reduction in free surface energy. In pairings where one surface energy is high and one surface energy is low the crucial factor is very often the opportunity for interactive effects, for example, when water is applied to a hydrophobic surface it is impossible to bring about any noticeable reduction in surface energy. This is evident in that the wetting is poor. The water applied forms droplets with a very high contact angle. Perfluorinated hydrocarbons, e.g. polytetrafluoroethylene, have very low surface energy. There are hardly any components which adhere to surfaces of this type, and components deposited on surfaces of this type are in turn very easy to remove.
The use of hydrophobic materials, such as perfluorinated polymers, for producing hydrophobic surfaces is known. A further development of these surfaces consists in structuring the surfaces in the μm to nm range. U.S. Pat. No. 5,599,489 discloses a process in which a surface can be rendered particularly repellent by bombardment with particles of an appropriate size, followed by perfluorination. Another process is described by H. Saito et al. in “Service Coatings International” 4, 1997, pp. 168 et seq. Here, particles made from fluoropolymers are applied to metal surfaces, whereupon a marked reduction was observed in the wettability of the resultant surfaces with respect to water, with a considerable reduction in tendency toward icing.
U.S. Pat. No. 3,354,022 and WO 96/04123 describe other processes for reducing the wettability of objects by topological alterations in the surfaces. Here, artificial elevations or depressions with a height of from about 5 to 1000 μm and with a separation of from about 5 to 500 μm are applied to materials which are hydrophobic or are hydrophobicized after the structuring process. Surfaces of this type lead to rapid droplet formation, and as the droplets roll off they absorb dirt particles and thus clean the surface.
This principle is borrowed from the natural world. Small contact surfaces reduce Van der Waals interaction, which is responsible for adhesion to flat surfaces with low surface energy. For example, the leaves of the lotus plant have elevations made from a wax, and these elevations lower the contact area with water. WO 00/58410 describes these structures and claims the formation of the same by spray-application of hydrophobic alcohols, such as 10-nonokosanol, or of alkanediols, such as 5,10-nonokosanediol. A disadvantage here is that the self-cleaning surfaces lack stability, since the structure is removed by detergents.
Another method of producing easy-clean surfaces has been described in DE 199 17 367 A1. However, coatings based on fluorine-containing condensates are not self-cleaning. Although there is a reduction in the area of contact between water and the surface, this is insufficient.
EP 1 040 874 A2 describes the embossing of microstructures and claims the use of structures of this type in analysis (microfluidics). A disadvantage of these structures is their unsatisfactory mechanical stability.
JP 11171592 describes a water-repellent product and its production, the dirt-repellent surface being produced by applying a film to the surface to be treated, the film having fine particles made from metal oxide and having the hydrolysate of a metal alkoxide or of a metal chelate. To harden this film the substrate to which the film has been applied has to be sintered at temperatures above 400° C. The process is therefore suitable only for substrates which are stable even at temperatures above 400° C.